[~]$ perl -le '$re = qr/<!--.*?-->/; print $re;'
(?-xism:<!--.*?-->)
[download]

As you can see, the qr// specificly turns off all options for the regex that you don't specify, which is why adding the /s afterwards doesn't make any difference.

As for whether this is a good thing or not, I can see arguments either way.

• For: When you created the RE, you didn't say you wanted that option, so Perl makes sure that this particular part of the RE doesn't have that option. This is probably more usful when building a large RE up from smaller parts, and you want the different parts to act differently with regards to the various options.
• Against: Like you saw, adding the /s didn't do what you meant. And Perl, after all, is usually very DWIMy.

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